Locomotive firebox and bracing means therefor



Dec. 30, 1952 A. TROSS 2,623,508

LOCOMOTIVE FIREBOX AND BRACING MEANS THEREFOR Filed Nov. 23, 1949INVENTOR ARA/OLD TROSS AGENTS Patented Dec. 30, 1952 LOCOMOTIVE FIREBOXAND BRACING MEANS THEREFOR Arnold Tross, Munich, Germany ApplicationNovember 23, 1949, Serial No. 128,952 In France September 10, 1948Claims.

The invention relates to the firebox portion of locomotive boilers,particularly of the Stephenson design. In such a firebox, the outer andthe inner shells are connected by crown stays and cross stays on theupper side, by staybolts on the lateral sides, and by the foundationring at the bottom. The task of the stays and foundation ring is topreserve the shape of the firebox under the stress of high steampressure and high temperature, and to prevent leakages within thevarious joints as well as fractures of the various structural elements.This task is difficult to perform, since the inner shell tends to expandmore than does the outer shell, because of the much higher, and alsoless uniform, temperature of the former.

Until recently, the staybolts used for connecting the side sheets weremostly of the rigid type, within the peripheral zones also of theflexible type, with their rigid ends screwed into the sheets and sealedby over-size threads, expanding, and riveting. Sometimes, riveting waslater replaced by sealwelding the protruding end of the bolt head to thesurface of the sheet, particularly in the case of steel fireboxes. Morerecently, the thread was abandoned in order to simplify installation andto save tools; plain cylindrical bars or bolts with cylindrical headswere used instead, with their protruding ends fastened to the fireboxsheets by welding. The clearance between the staybolt and thecorresponding hole, which is necessary for the installation, was kept assmall as possible.

The known methods of securing the staybolts to the firebox sheets havenot proved satisfactory. The main defects are that, after a short periodof service the staybolts first begin to show incipient fissures, then tocorrode, and at last to break; the side sheets of the firebox begin tobulge in an outward direction, and, starting from the edges of the drillholes and extending radially into the sheets, cracks begin to appear inthe most heavily strained portions of the firebox, particularly in theso-called fire zone. The cracks and fractures necessitate frequentreplacement of the damaged staybolts and wall sections, and keep theengine out of service for a considerable length of time.

The present invention is based on the knowledge of the cause of thefailures and defects described, and provides meanstopreventthese eithercompletely or very nearly so. Based on thorough preliminary calculationand practical observation, by the inventor, of the mutual interaction ofthe connected structural elements, it was rec- 2 ognized that the designand the connection of the structural elements of the firebox as hithertoused are not sufficient to prevent permanent deformations which rapidlylead to fatigue failures.

The main cause of the progressing permanent deformation consists in thefollowing process: the lower parts of the side sheets of the innerfirebox are particularly exposed to the heat. They are prevented,however, from expanding according to the rise in temperature by beingfirmly connected to parts of considerably lower temperature; i. e. tothe solid foundation ring by a rivet seam; to the outer side sheets bymeans of the staybolts; and to door sheet and tube sheet by theirflanges.

By thus preventing the thermal expansion, a state of stress is caused,which the applicant has termed unrelieved thermal expansion stress. Thisstress leads to plastic deformation of the individual areas between thestaybolts, as well as'of the entire firebox unit-a deformation'termed'as bulgingwhich also affects the foundation ring and mostly works in anoutward direction. The forces caused by the boiler pressure on the outerand inner shells result, as the area of the outer shell is usuallylarger, in an additional outwardly directed component.

In view of these considerations, the invention shows a new way ofconstruction of a firebox which is fundamentally new. Measures ofdifierent kind are provided as follows:

(a) The thermal expansion stress is eliminated as the chief reason forthe described constant deformations of the construction elements at thepoints of the boiler connected to each other.

(b) The construction parts connected to each other and their connectionsare so shaped that the remaining expansions are taken up elastically.

The result is that in spite of the stresses by the boiler pressure andheat acting during long periods of time the service life of thecorresponding walls and of the staybolts is materially increased.

By these measures, the economy and, above all, the safety of locomotiveoperation are decisively increased, and the hitherto inevitable waste ofmaterial is prevented. Besides, they result in considerable savings inlabor, tools, and other means for conservation and repair work.

The means applied by the invention to prevent permanent deformation ofthe structural elements of the firebox are exclusively means of design.Some of them refer to the walls, some to the foundation ring, most ofthem, however, to

the staybolts and the way of fixing these to the wall sheets. Theysupplement each other and may be applied singly or simultaneously.

Accordingly, the invention aims at an immediate reduction of thestresses within the staybolts by improving their design as well as themethod of their installation. This is based on the perception'thatfailures of 'staybolts may cause heavy strains to the correspondingareas of the shells.

The accompanying drawing shows a staybolt, characterized by cylindricalheads l, 2 and a diameter decreasing towards the cylindrical-middleportion of the shaft, which is rigidly fixed into the firebox bywelding. According to one means of the invention, the difference betweenthe diameter 3 of the bolt head and the diameter 4 of the correspondinghole ought to be not 'less than 0.5 mm. and preferably not larger than 2mm. at least on the side of the inner shell 9. This space takes up thatamount of thermal expansion by which the thermal expansion within thefire zone of the inner shell 9 exceedsthe-expansion'of the-outer shellIt. Disturbing effects between boltlhead and plate as caused byunrelieved thermal expansion stresses and leading to permanentdeformations or even, when the firebox cools down, to cracks at theedges of the holes, are thus safely prevented.

'This measure represents a fully intentional deviation from the commonpractice of fitting the bolt as tightly into the hole as theinstallation permits. Relieving the thermalexpansion stress alsoprevents bulging of the single stay-bolted areas, and reduces bulging ofthe side sheet a a whole. This, in turn, considerably reduces theS-shaped deflection of the staybolts which results fromthelateralshifting of the stay ends, .relative :to each other, in theperipheral zones of the bulgingsheets. Both effects substantially reducethe bending stresses in the sheets and in the stays, and preventincipient cracks and fractures;

Preferably according to the invention, the shape of the tapered portionsof the staybolt shaft is :such that their maximum diameter 5 differsfrom the minimum diameter tby 4 to 8 mm., and that their length i,contrary to common practice, increases with the length 8 of the shaft.The value of the difference between the diameters dependson thecircumstances, 1. e. material used for and diameter of the head of thestaybolts. The length of one taper ranges from 0.25 to 0.45 of the totallength 8 of the shaft,with the greater length ofthe taper referring tothe greater-diameter difierences. staybolts .most commonly used havecylindrical shafts, whiohare sometimes even threaded over thelength. .Asregards the relation between the length of the tapered sections and theentire staybolt, it must be emphasized that, with the fewtaperedstaybolts hitherto known, the tapered length is independent of the totallength, and is the same for the whole set of staybolts, of which thelengths vary between approximately 4 and 16 inches for the same boiler.In the design accord- .ing to the inventionthe higher of the givenvalues refer to the greater diameter differences. For a given differencebetween maximum :and minimum diameters of the tapered portion, there isan approximately constant proportionality be- 1 tween the lengthof thetapered portion and the total length of the shaft, independent of thelength-of theentire staybolt.

'The new design simultaneously increases the resilience of thestaybolt,i. e. the strain energy As compared with this type,

4 the staybolt can absorb without exceeding the yield point at operationtemperatures. This is extremely important in order to preserve the shapeof the firebox as calculation and observation have revealed that thedeformation of the firebox begins with a permanent deformation of thestaybolts. These form, in a way, the backbone of the firebox.

In order'to 'compensate for the loss in strength caused by the weldingheat in the vicinity of the weld seam, and thus to prevent permanentdedesign, of the tapered section at its largest diameter.

If Tb is'the bending tension, Mb the bending momentum and W the momentumof resistance, the following formula must be applied:

Of the various means suited to achieve, ac-

cording to the invention, the effect of inten- W being equal tionallyinfluencing, and particularly of reducing, the permanent deformationfor" the structural elements, only some which maybe considered, the mosteffective have been mentioned, without the invention being in anyway'confined to these.

I-claim:

1. In a locomotive boiler having inner and outer firebox shells,staybolts with heads rigidly fastened in the shells :of the .firebox andwith a shaft tapering in diameter fromizheirigidly'fastenediheadsito.th'e cylindricalmiddle portion in such a waythat theratio 'Lr/Lsbetween the tapering length 'Leand the total length of the shaft Ls is,for all staybolt lengths. occurring in the firebox, approximatelyconstant for-a given difference (Dmax-Dmin) between the maximum diameter(Dm'ax)-'a,1'1d. the minimum diameter '(Dmm) o'f thetapered portion,*thenumerical value ofthe ratio Lt/Ls being at least 0.25 and at theuttermost 0.45 and the difference between Dmax and Dmin at least 4mm.and at the uttermost -9 mm.

2. Staybolts according to claim '1, the ratio Li/Lsfor all stayboltlengthsoccurring in the fire-- box being mill-.475 Dmax' 0.035Balm-0.137), m being a factor the numerical value of which may bebetween 0.9 and.1.1 forrigidly fixed staybolts.

.3..In a locomotive boiler having inner and outershells, staybolts withheads weld-fixed at .least'to the inner .shelland larger'in diameterthan the cylindrical;middle portion of the shaft, wherein the length ofthe 'one'head connected to the inner shell is about that needed forrigidly fastening the staybolt to the inner shell, whilst the length ofthe other head connected to the outer-shellis greater'than that of theone head as far-as is necessary, having-regard to any relativeinclination between the inner and outer shells and to theeconomicallimitation in the number of different staybolt sizes to be provided,said shaft tapering in diameter from the rigidly 'fastened heads to "thecylindrical middle portion in such a way that the ratio lit/ Ls'betweenthe tapering length L1; and the total length of the shaft Ls is for'allstaybolt lengths :occurring in the firebox, approximately constant for agiven difference (Dinar-Darin) between 'themaximumdiameter (Daisey-andthe minimum diameter (Dmln) of the tapered portion, the numerical valueof the ratio LIE/LS being at least 0.25 and at the uttermost 0.45 andthe difference between Dmax and Dmin at least 4 mm. and at the uttermost9 mm.

4. In a locomotive boiler having inner and outer shells, staybolts witha shaft and heads, said heads and adjacent portions of the shaft beinglarger in diameter than the cylindrical middle portion of the shafttapering in diameter from the rigidly fastened heads to the cylindricalmiddle portion in such a way that the ratio Lt/Ls between the taperinglength Lt and the total length of the shaft Ls is, for all stayboltlengths occurring in the firebox, approximately constant for a givendifference (Dma.x-Dmln) between the maximum diameter (Dmax) and theminimum diameter (Dmln) of the tapered portion, the numerical value ofthe ratio Lt/Ls being at least 0.25 and at the uttermost 0.45 and thedifference between Dmax and Dmln at least 4 mm. and at the uttermost 9mm, at least the heads secured to the inner shell being Welded thereto,the welded heads being of such shape and dimensions in relation to thoseof the staybolt shaft as to have a momentum of resistance to bending atleast 30% greater than that of the shaft at its largest crosssection.

5. In a locomotive boiler having inner and outer shells, staybolts witha shaft and heads secured in holes provided in the shells, the head 6 ofeach staybolt secured to the inner shell being Weld-fixed and larger indiameter than the cylindrical middle portion of the shaft, the weldedheads having an all round clearance at least in the holes of the innershell, such clearance being provided with a difference in diameterbetween the heads and the appertaining holes of not less thansubstantially 0.5 mm.

ARNOLD TROSS.

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